1. Field of the Invention
The present invention relates to a piezoelectric element and a method for manufacturing the same. In particular, the present invention relates to, for example, an energy-trapping type piezoelectric element which uses thickness extensional vibration with respect to an oriented ceramic and a method for manufacturing the same.
2. Description of the Related Art
Energy-trapping type piezoelectric element are known. In this type of element, vibration electrodes are arranged in a piezoelectric body, and high-order vibration of the thickness extensional vibration is excited. It is clear that this type of piezoelectric element is hardly affected by the Poisson ratio in contrast to that which excites fundamental vibration of the thickness extensional vibration. In general, a material which exhibits excellent thermal stability, such as a lead titanate-based material, has a Poisson ratio of less than ⅓, and frequency-lowering type energy-trapping of a fundamental of the thickness vibration is impossible. For a high-order vibration of the thickness vibration excited by the structure of the vibration electrodes, excellent frequency-lowering type energy-trapping can be realized even when such a thermally stable material is used, and therefore, this technology is noted being capable of providing a commercial high-performance piezoelectric element.
In addition, it is known that an oriented ceramic, i.e., one in which crystal grains are oriented, is useful as the ceramic used in an electronic material, for example, a piezoelectric material. Orientation refers to a condition in which at least one axis of each of the crystal grains is preferentially aligned in the same direction as a whole. For example, as is clear from the report by T. Takenaka et al., regarding a piezoelectric material, the electromechanical coupling coefficient is increased to about 2.2 times that of a usual randomly-oriented ceramic in longitudinal vibration of a cylindrical vibrator by bringing a layer perovskite compound ceramic, for example, Na0.5Bi4.5Ti4O15, into orientation (Sensor and Materials, Vol. 1, 35 (1998)). As is reported by S. Jin et al., regarding a superconducting material, the critical current density is increased to about 12 times that of a randomly-oriented ceramic by preparing an oriented ceramic of YBa2Cu3O7-δ (Physical Review B, vol. 37, No. 13, 7850 (1988)).
Examples of conventional methods for manufacturing an oriented ceramic include, for example, a hot-forging method and a Templated Grain Growth (TGG) method. T. Takenaka et al. prepared the oriented Na0.5Bi4.5Ti4O15 ceramic by using the hot-forging method. Hot-forging is a method in which a molding is heat-treated (fired) under pressure. An oriented ceramic having a high degree of orientation can be produced by this method. At this time, the degree of orientation of the oriented ceramic reaches 98% when measured by the Lotgering method. Seong-Hyeon Hong et al. prepared an oriented Bi4(Ti3.96Nb0.04)O12 ceramic by using the TGG method. The TGG method is a method in which ceramic crystal grains having anisotropy are mixed in advance of molding. The degree of orientation of the oriented ceramic produced by this method is 96% when measured by the Lotgering method, and the piezoelectric constant d33 is increased to about 1.5 times that of a randomly-oriented ceramic (J. Am. Ceram. Soc. Vol. 83, 113, (2000)).
Conventionally, it was difficult to manufacture the energy-trapping type piezoelectric element in which vibration electrodes were arranged in a piezoelectric body, as high-order vibration of the thickness vibration was excited, and the piezoelectric ceramic used therefor had a polarization axis present in the major-axis direction of the crystal grain having shape anisotropy, while the crystal grain was oriented in the direction perpendicular to the vibration electrode surface. In particular, it was easy to manufacture a ceramic compound having a layer perovskite structure, in which the c axis of the orthorhombic system was preferentially oriented by using the hot-forging method, the TGG method and the like. However, the direction suited for polarization is the a axis, and it was difficult to form vibration electrodes in the piezoelectric body in the direction perpendicular to the a axis in order to apply an excitation electric field in the direction of spontaneous polarization.
For example, since uniaxial pressure is applied during firing in the hot-forging method, the resulting molding is crushed and is significantly deformed. Consequently, even when the electrodes were arranged beforehand in the piezoelectric body, deviation from the original location of the electrodes was significant, reducing the ability to the element to be manufactured by the hot-forging method.
In the TGG method, a green sheet is prepared by a sheet molding method, electrodes are printed on the green sheet, and thereafter, the green sheets are laminated, followed by pressure bonding, so that the electrodes can be precisely formed in the piezoelectric body. However, since the c axis orientation is brought about in the direction perpendicular to the traveling direction of a carrier film during sheet molding in the TGG method, the preferentially oriented a(b) axis and the vibration electrodes become perpendicular to each other. In the TGG method, the c axis is an axis unsuited for polarization, and the remnant polarization is very small. Therefore, there is a problem in that the piezoelectric property is hardly attained.